Charging Up Random Access Memory

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Advantages of ferroelectric memory include lower-power consumption, fast write operations, and durable storage, Liu says. The MIT-Tsinghua results show the separation of positive and negative charges, or polarization, in their sample was in-plane, or parallel, with the atomically flat sample, creating a potential change on the edges of square-shaped islands of the material. Since this potential difference along edges is measurably different, one with large tunneling current, the other small, it can realize two different states that represent either a zero or a one, and these states can be detected simply by measuring the current.

“Based on this property, we proposed a new kind of random access memory. We call it ferroelectric tunneling random access memory,” says Liu, who proposed the initial architecture for this kind of memory, along with Fu and three coauthors at Tsinghua University: Kai Chang, Xi Chen, and Shuai-Hua Ji. MIT has filed for provisional patent protection and is in the process of filing a utility covering the findings regarding in-plane polarization and tunneling current. “It’s very simple, and it’s really practical, and I think it could be realized in the near future,” Liu says.

Previous conventional capacitive ferroelectric random access memory technologies had to destroy a state to read it, Liu says, which meant an extra step of rewriting the information stored in memory after reading it. “In our case, we read the signal without destroying it,” Liu says. “This is the intrinsic advantage of our approach. … Therefore it can have much higher read operation performance.”

“In our experiments, we found that ferroelectricity persists for the very small islands, as small as 25 nanometers by 25 nanometers by 0.5 nanometers; even in these very small islands, the ferroelectricity persists. We could achieve much higher storage density because it is really small,” he explains.

“The authors and their collaborators use a state-of-the-art combination of molecular beam epitaxy and scanning tunneling microscopy to demonstrate a completely unexpected enhancement of ferroelectricity in ultrathin films of [tin-tellurium],” comments Ilija Zeljkovic, an assistant professor of physics at Boston College, who was not involved in this research. “This discovery can potentially be employed in nanodevices, such as the ferroelectric RAM nanodevice the authors describe [in Figure 4 of the Science report].”

Although the 3-D bulk form of tin-tellurium has been studied for decades, the new results in ultrathin 2-D film of the same material exhibit this surprising new phenomenon, Zeljkovic notes. “The study itself is extremely thorough, and the data presented is of the highest quality in spite of the high difficulty of the experiment performed. The study also highlights the recent effort in the condensed matter physics community to search for novel interface phenomena in ultrathin films of existing materials, for example, graphite versus graphene.”

Fewer tin vacancies

Mathematical calculations known as density functional theory matched the experimental findings that there are 1/20 to 1/30 as many tin vacancies in the atomically thin tin-tellurium film than in the bulk form of the material. This lack of defects is believed to contribute to formation of the ferroelectric state.

The next step will be to show these results in actual devices. Future challenges include how to easily and inexpensively produce high quality tin-tellurium thin films and how to precisely control the polarization direction.

The research was supported by the U.S. Department of Energy, the STC Center for Integrated Quantum Materials, the National Natural& Science Foundation, and Ministry of Science and Technology of China.